Cellulose esters are valuable polymers that are useful in many plastic, film, coating, and fiber applications. Cellulose esters (CEs) are typically synthesized by the reaction of cellulose with an anhydride or anhydrides corresponding to the desired ester group or groups, using the corresponding carboxylic acid as diluent and product solvent. Some of these ester groups can afterward be hydrolyzed to obtain a partially-esterified product. These partially substituted cellulose esters have great commercial value, and find use in coatings, where their greater solubility and compatibility with co-resins (in comparison with triesters) and hydroxyl group content (to facilitate crosslinking) are prized.
An important aspect in obtaining suitable cellulose esters has traditionally been maintaining molecular weight during the esterification process. A loss in molecular weight is associated with poor plastic properties and brittle films, a flexible film being the desired goal. Thus, it has long been recognized that in order to obtain a suitable chloroform-soluble (triacetate) cellulose ester, the acetylation process must not result in significant degradation, or lowering of the molecular weight, of the cellulose. See, for example, U.S. Pat. No. 1,683,347.
When it was discovered that these early triacetate esters could be modified, via partial hydrolysis of the acetate groups, to obtain acetone-soluble cellulose acetate, maintaining a suitable molecular weight during hydrolysis remained important. See, for example, U.S. Pat. No. 1,652,573. It was recognized as early as the 1930's that the amount of hydrochloric acid present in the reaction mixture during partial ester hydrolysis must be carefully controlled to avoid hydrolysis or breakdown of the cellulose acetate. See, for example, U.S. Pat. No. 1,878,954.
Likewise, U.S. Pat. No. 2,129,052 advised that hydrolysis under severe conditions such as high temperature or high concentration of catalyst caused degradation of the cellulose, the resulting products being unsuitable for commercial use because of their low strength. U.S. Pat. No. 2,801,239, relating to the use of zinc chloride as an esterification catalyst, cited as an advantage that the process minimized the rate of breakdown of the cellulose. U.S. Pat. No. 3,518,249 acknowledged that little interest had been shown in cellulose esters of an extremely low degree of polymerization. More recently it was confirmed that the rate of hydrolysis in cellulose esters is controlled by temperature, catalyst concentration, and, to a lesser extent, by the amount of water, and that higher water content slightly increases the rate of hydrolysis and “helps minimize degradation.” Kirk-Othmer, Encyclopedia of Chemical Technology, Fourth Ed., vol. 5, pp. 496-529, 509 (1993), John Wiley & Sons, New York, N.Y.
When used in coating compositions, conventional cellulose esters provide many benefits, including improved hardness, improved aluminum flake orientation, high clarity, high gloss, decreased dry-to-touch time, improved flow and leveling, improved redissolve resistance, reduced cratering, and reduced blocking. However, the performance properties of conventional cellulose esters are accompanied by an increase in viscosity, which must be offset by increasing the level of solvents used. With recent concerns of VOC levels in coating compositions, there remains a need for a cellulose ester product that provides the benefits of conventional cellulose esters, while providing only a moderate increase in viscosity without the addition of organic solvents. It would clearly be an advance in the art to provide cellulose esters that provide the performance properties of conventional cellulose esters, without an undue increase in viscosity when incorporated into coating compositions.
Although maintaining the molecular weight of cellulose esters during esterification and partial hydrolysis has long been deemed important in obtaining a suitable product, there has nonetheless been occasional mention in the literature of lower molecular weight cellulose esters.
For example, U.S. Pat. No. 3,386,932 discloses a method for reducing the molecular weight of cellulose triacetate with a catalyst such as boron trifluoride, the resulting bifunctional, low molecular weight cellulose triacetate then being used to produce linear block copolymers. This disclosure emphasizes the importance of maintaining the ester substitution at the 2-, 3-, and 6-positions of the triacetate, that is, wherein substantially all of the hydroxyl groups of the cellulose have been esterified, so that the hydroxyl functionality necessary for formation of the linear block copolymers preferentially appears only on the ends of the polymer chains.
U.S. Pat. No. 3,391,135 discloses a process in which hydrogen halides are used to reduce the molecular weight of cellulose derivatives. The examples describe methylcellulose powder and methyl-hydroxypropyl cellulose reacted with hydrogen chloride to reduce the molecular weight, as evidenced by a reduction in viscosity.
U.S. Pat. No. 3,518,249 describes oligosaccharide tripropionates, having an average degree of polymerization of from about 4 to about 20 and low levels of hydroxyl, that are useful as plasticizers and as control agents for the manufacture of foamed plastics. The oligosaccharide tripropionates are prepared by degrading a cellulose propionate in the presence of an acid catalyst. The patentees acknowledge that it has been an object in the art to provide methods of preventing the degradation of cellulose esters into low-viscosity oligosaccharide esters.
U.S. Pat. No. 4,532,177 describes base coat compositions that include a film-forming resin component, selected from alkyd, polyester, acrylic and polyurethane resins, from 1.0 to 15.0% by weight pigment, and from 2.0% to 50.0% by weight of a cellulose ester material. The '177 patent suggests a solution viscosity for the cellulose ester material from 0.05-0.005 seconds, an acetyl content from 10.0-15.0% by weight, a propionyl content from 0.1-0.8% by weight, a butyryl content from 36.0-40.0% by weight, and a free-hydroxyl content of from 1.0-2.0% by weight. However, the examples of the '177 patent use a cellulose ester having a solution viscosity of 0.01, which is approximately equivalent to an inherent viscosity (IV) for such an ester of from about 0.25 to about 0.30 dL/g, as measured in a 60/40 (wt./wt.) solution of phenol/tetrachloroethane (PM95) at 25° C. We have found that solution viscosities less than about 0.01 correlate poorly with IV values and GPC molecular weight values, although there is a strong correlation between IV and GPC molecular weights.
WO 91/16356 describes a process for the preparation of low molecular weight, high-hydroxyl cellulose esters by treating a cellulose polymer with trifluoroacetic acid, a mineral acid, and an acyl or aryl anhydride in an appropriate carboxylic solvent, followed by optional in situ hydrolysis. The cellulose esters obtained according to the disclosure are said to have a number average molecular weight (Mn) ranging from about 0.01×105 (about 1,000) to about 1.0×105 (about 100,000), and an IV (inherent viscosity) from about 0.2 to about 0.6, as measured at a temperature of 25° C. for a 0.25 gram sample in 100 ml of a 60/40 by weight solution of phenol/tetrachloroethane.
Japanese Kokai Patent Publication No. 51-119089 describes a process for the preparation of a low molecular weight cellulose mixed organic acid ester that involves heating cellulose acetate with a saturated or unsaturated organic acid of 3 or more carbon atoms (propionyl or higher), in the presence of an acid catalyst, with removal of the resulting acetic acid from the reaction mixture, to obtain a lower molecular weight cellulose mixed organic acid ester. The starting material for this process is cellulose acetate.
Another patent document naming the same inventors, Japanese Kokai Patent Publication No. 51-119088, discloses a method for the manufacture of a low molecular weight cellulose organic acid ester that includes heating cellulose acetate with a saturated or unsaturated organic acid at a temperature above 30° C. in the presence of a cation exchange resin, the resulting ester having a lower molecular weight than the starting material. The starting material for the disclosed process is cellulose acetate.
Both of these references teach low molecular weight mixed cellulose esters. The process uses cellulose acetate as starting material, and performs a transesterification while hydrolyzing the cellulose backbone, the amount of higher mixed ester introduced being relatively low.
U.S. Pat. No. 6,303,670 discloses an ultraviolet-curable cellulosic coating composition comprising a cellulose acetate, a diepoxy compound, and a photo cationic polymerization catalyst. The cellulose acetate useful in these compositions is a low molecular weight cellulose acetate, having a number-average molecular weight of from 1,500 to 5,000, and is prepared from cellulose triacetate by hydrolysis. According to this disclosure, the degree of substitution of hydroxyl groups must be from 1 to 3, since hydroxyl values of less than 1 are said to result in insufficient crosslinking in the final coating composition.
Although efforts have been made to prepare oligosaccharides via stepwise addition of anhydroglucose units, these methods are not believed to result in cellulose derivatives that are suitable for coating applications. Further, the costs of such processes would be significant. See, for example, Nishimura, T.; Nakatsubo, F. “Chemical Synthesis of Cellulose Derivatives by a Convergent Synthetic Method and Several of Their Properties,” Cellulose, 1997, 4, 109. See also Kawada, T.; Nakatsubo, F.; Umezawa, T.; Murakami, K.; Sakuno, T. “Synthetic Studies of Cellulose XII: First Chemical Synthesis of Cellooctaose Acetate,” Mokuzai Gakkaishi 1994, 40(7), 738.
The present applicants have unexpectedly discovered that relatively low molecular weight cellulose mixed esters, which were thought to lack the properties necessary to provide the performance characteristics of conventional molecular weight esters, can be incorporated into coating compositions, without an undue increase in viscosity, and without the high levels of solvent heretofore necessary in the preparation of high solids coatings containing cellulose esters. Also surprisingly, the properties of the resulting coatings, when the coating compositions are applied and cured, are comparable in most respects to those made using conventional molecular weight esters.
Various esters according to the invention exhibit improved solubilities in a variety of organic solvents, compatibility with various co-resins, and suitable melt stability after prolonged exposure to melt temperatures. Further advantages of the inventive esters are set forth in the following.